1. Field of the Invention
The present invention relates to nitride semiconductor devices comprising an active layer containing indium such as nitride semiconductor laser devices and laser diodes, light emitting diodes (LEDs), photodetectors, high-frequency transistors, and high-voltage transistors. More specifically, it relates to nitride semiconductor lasers and nitride semiconductor devices having improved device characteristics as the result of using a nitride semiconductor substrate having a specific off angle.
2. Background Information
Laser devices comprising nitride semiconductors such as GaN semiconductors can be used at emission wavelengths ranging from the ultraviolet region of 370 nm or less to wavelengths in the region of 500 nm or more. Substrates on which such laser devices are formed are grown using a lateral growth technique known as Epitaxial Lateral Overgrowth (ELO).
This is a technique wherein GaN is grown laterally to form a GaN layer on an underlying substrate on which a SiO2 mask pattern having periodic stripe openings is formed. Next, the underlying substrate is removed to produce a substrate comprising only the GaN layer. The GaN in the laterally grown region constitutes a low-dislocation (or defect) region. As the resulting low-dislocation GaN has good crystalline characteristics, the life of nitride semiconductor laser devices using this GaN as a substrate can be increased.
Furthermore, there is a demand for improved device characteristics in nitride semiconductor laser devices. Consequently, a substrate is required wherein not only is the dislocation density of the substrate itself reduced, but on which it is possible to grow a nitride semiconductor layer having good crystalline characteristics. Moreover, in order to make practical use of the nitride semiconductor device, it is necessary that the diameter of the substrate itself be increased.
Consequently, a substrate has been proposed that comprises a hexagonal system nitride semiconductor and forms an off angle, in a predetermined direction, of no less than 1° and no greater than 20° from the (0001) plane (see, for example, Japanese Unexamined Patent Application JP-2002-016000-A).
However, the broad ranges proposed for the size and direction of this off angle for the nitride semiconductor substrate resulted in an uneven composition in indium, aluminum and the like, and an uneven distribution of impurities and the like within the laser device. In particular, in the case of laser devices comprising an active layer containing indium, depending on the emission wavelength, this may lead to increases in the threshold current. Nitride semiconductor laser devices are theoretically capable of laser emission over a broad range of wavelengths; in practice however, if compositional distribution, such as the distribution of indium in the active layer, is not uniform, problems arise such as the inability to achieve this broad emission range.
Furthermore, when preparing substrates having large diameters of greater than one inch, as pits and grooves remain on the principal face of the substrate, there are problems in that it is necessary to flatten the surface produced by forming the nitride semiconductor layer on the substrate.